1. Field of the Invention
The present invention relates to the use of a water blocking material in the form of a surface coating on a fiber. The substrate may be used in a fiber optic cable to prevent water ingression into the cable.
2. Description of the Related Art
Many processes for the treatment of substrates with water blocking materials are known. In particular fibers, fibrous materials or yarns are impregnated with water blocking materials in certain applications where water is undesirable.
For example, water blocking materials are used in fiber optic cables to prevent ingression and propagation of water in the cable. Fiber optic cables are normally made by surrounding wave guides with reinforcing fibers that prevent elongation of the cable, and those reinforcing fibers are then encased in plastic. If water enters the fiber optic cable, it migrates inside the cable, usually longitudinally by capillary action, until the water contacts the sensitive wave guides and ultimately optical network connection boxes. The wave guides are made of glass, and when contacted by water become dull. The efficiency of the signal transmission through the wave guide drops until the wave guides can no longer transmit a signal. When that happens, the damaged portion of the cable must be located and then replaced. Since these fiber optic cables may be buried underground or may be placed along the bottom of large bodies of water, the location and replacement of the damaged sections of the cable can be time consuming and expensive.
As a result of these problems, there have been developed numerous methods to protect the fiber optic cable from water ingression. One method has been to coat the surrounding reinforcing fibers with a water blocking material so that if water should leak through the plastic casing, that water will be absorbed by the water blocking material on the reinforcing fibers to prevent damage to the wave guides.
There are processes in which the treatment of fibers is carried out with water blocking material as an aqueous dispersion such as in EP-A-0 351 100. One disadvantage of this process is that the viscosity of these aqueous dispersions is very high.
In EP-A-0 666 243, a method is disclosed in which glass fiber bundles are treated with a dispersion of a water absorbing material in an oil, wherein poly(sodium acrylate) is the water absorbing medium. In the same way derivatives of polyacrylic acid are disclosed in WO93/18223 as super-absorbing materials. In both cases, the water blocking materials are used in water-in-oil emulsions for the treatment of substrates, with the super-absorbing materials being contained in the aqueous phase. These emulsions are, however, complicated to manufacture and require the use of emulsifying agents.
In general, conventional water blocking materials are based on cross linked polyacrylics and/or cross linked polyacrylates, such as water-in-oil emulsions. These types of water blocking materials all contain water and oil, and when the material is applied to a fiber or yarn some of the water and oil must be removed. The removal of water and oil is an additional processing step that is energy intensive, limits productivity and is an environmental burden.
What is needed, therefore, is a water blocking agent that is effective and is easy to apply to fibers.
None of the known water blocking materials, when applied to a substrate, completely meet the following four criteria defined as ideal industry standards and related manufacturing requirements:
First, the substrate coated with the superabsorbent polymer must be easily processable when used in manufacturing, for example, optical cables. This means that the reinforcing superabsorbent coated substrate must have good frictional properties and a low tendency to generate deposits when spiraled or braided around a core optical fiber. Conventional superabsorbent polymers used for the coating of substrates are known to be prone to deposits due mostly to the high stiffness of the film formed or their relatively large particle sizes, that is, above 40 microns.
Secondly, the residual water present in substrates coated with conventional superabsorbent polymers causes blistering, for example, during the extrusion of the outer layer jacket of an optical cable. Therefore, such coated substrate must be as dry as possible to avoid this blistering problem. For example, water in oil emulsions of superabsorbent polymers contain a substantial proportion of water (up to ⅓) which may cause blistering when substrates having high loadings of the superabsorbent polymers are extruded above certain temperatures and thus be detrimental to the overall quality of the cable.
Thirdly, the substrate coated with the superabsorbent polymer must withstand temperatures encountered during thermal processing, such as the extrusion process mentioned above. It is known in the art that most superabsorbent polymers do not tolerate temperature cycling and therefore lose their ability to absorb water. The detrimental chemical mechanism is generally associated with the formation of inter-chain anhydrides which does not constitute a good entrapping network for the “ingressing” water.
Fourthly, most aerial or liaison cables (sometimes referred to as riser cables) which connect aerial/underground cables to building networks, are exposed to freezing conditions in most regions where they are used. Therefore it is essential that the cable in general and the coated substrate in particular be resistant to freezing conditions. The formation of ice within the cable structure does not only affect the dimensional stability of the system but also may cause internal damage related to the reduced flexibility associated with a microbending crushing effect. Therefore, the strength of the coated substrate in the cable is very important and therefore its dryness is absolutely essential. Unfortunately a large number of known superabsorbent polymers not only contain water but also are not resistant to freezing conditions.
In view of the above, an object of the present invention is to provide a substrate coated with a water blocking material which meets the four criteria discussed above. Such substrate is highly processable, essentially water free and temperature tolerant, that is resistant to freezing and temperature cycling.